(1) Field of the Invention
The present invention relates to a fabrication process for semiconductor integrated circuits, and more specifically to the dry etching of titanium-tungsten films that are used as part of the integrated circuit.
(2) Description of Prior Art
The trend in the semiconductor industry has been an ongoing increase in circuit density, while the cost of advanced semiconductor chips decrease. This has been accomplished in part by the ability of the semiconductor engineering community to decrease critical chip dimensions, and thus produce more circuits per chip. The major factor in the reduction of feature size has been the continued development of photolithographic equipment and materials. Advances in cameras, as well as the creation of more sensitive photoresists have allowed critical chip features to be reduced. The steady development of reactive ion etching tools and processes, enabling the small critical images in photoresist to be transferred to the underlying layers, have also played a major role in chip size and cost reduction.
However specific semiconductor processes and materials, that have been used in the past for chip manufacturing, are now not as compatible with the smaller chip features as they once were when larger dimensions were being used. For example aluminum, or aluminum based metallizations, have been an industry standard for all levels of interconnects. However the decreasing features may suggest a change to a different metallization process. If a contact hole to a silicon device region, or a via hole between two levels of wiring, is drastically reduced in size, the use of aluminum may result in electromigration failures. The current density of the aluminum in the narrow opening may become so large that metal migration occurs. Another shortcoming of aluminum, in reference to smaller device or via holes, is the difficulty in filling these openings, with standard aluminum deposition techniques, particularly when the aspect ratio is high. Therefore the industry has looked to other metallization processes to accomadate the advancement to chip miniturazation.
The use of refractory metals, such as tungsten, or alloys of tungsten, such as titanium--tungsten, (TiW), are now being developed and used, in combination with, or as a replacement of aluminum, as described by Marangon, in U.S. Pat. No. 5,407,861. One major advantage of tungsten or TiW, is the ability to withstand very high current densities without the risk of electromigration failure. Therefore even with submicron dimensions, the current density in these contact or via holes, when using refractory metallizations, would not present a problem. Also chemical vapor deposition processes can be employed to deposit these films, thus resulting in a more conformal deposition than the marginal conformality resulting from the evaporated aluminum process. In addition to the electromigration resistance advantages offered by tungsten or TiW, another major advantage is the absence of aluminum penetration into the device region when used as the contact device metallization. When these refractories are used as the underlying layer for an aluminum--TiW metalization, they will prevent aluminum from reaching and penetrating the silicon device region. Thus the direction to refractory contact metallizations, in place of aluminum, has occurred.
One area needed to be developed to accommodate refractory metallizations is the etching or patterning of this metallization. Solutions have been offered. U.S. Pat. No. 4,980,018 by Mu et al, and U.S. Pat. No. 4,997,520 by Jucha et al, show processes that enable the reactive ion etching of tungsten to be accomplished. However applications in which composites of overlying aluminum and underlying TiW, are to be patterned, with the use of photoresist as a mask, are not disclosed. When plasma etching these composites, and using photoresist as the defining pattern, undercutting or tapering can occur. In addition selectivity between the masking photoresist and the composite metals can be low, sometimes resulting in severe erosion of the photoresist image during the etching procedure. Therefore the purpose of this invention is to achieve the desired profile in a aluminum--TiW layer, without undercutting or tapering, when using photoresist as the mask.